device/fido/mac: invalidate LAContext before deleting it

This causes any pending Touch ID consent prompt to be dismissed when the
authenticator is deleted, e.g. because the transaction was cancelled.
Otherwise the dialog sticks around until the OS lets it time out.

(See https://developer.apple.com/documentation/localauthentication/lacontext/1514192-invalidate?changes=_2.)

Bug: 678128
Change-Id: Ic4102b54ec78781ed869fa6a7827885ace5345e1
Reviewed-on: https://chromium-review.googlesource.com/1137218Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#575729}

device/fido/mac/touch_id_context.mm

@@ -34,7 +34,12 @@ TouchIdContext::TouchIdContext()
       callback_(),
       weak_ptr_factory_(this) {}
 
-TouchIdContext::~TouchIdContext() = default;
+TouchIdContext::~TouchIdContext() {
+  // Invalidating the LAContext will dismiss any pending UI dialog (e.g. if the
+  // transaction was cancelled while we are waiting for a fingerprint). Simply
+  // releasing the LAContext does not have the same effect.
+  [context_ invalidate];
+}
 
 void TouchIdContext::PromptTouchId(std::string reason, Callback callback) {
   callback_ = std::move(callback);